Thermoluminescent detector for mass spectrometer

ABSTRACT

A magnetic field is used to deflect and separate the charged particles of an ion beam to impinge on successive areas of a long dielectric crystal according to the charge-mass ratio of the particles. After exposure to the deflected ions, a heated wire is moved over the crystal to produce thermoluminescence in the crystal that is detected with a bank of phototubes. The resulting signals from the phototubes are correlated with the areas of the crystal and counted to indicate the spectrum of the particles.

United States Patent 1 on 3,812,354 Walter 1 May 21,1974

THERMOLUMINESCENT DETECTOR FOR 3,522,429 8/1970 Habfast 250/299 MASSSPECTROMETER Inventor: Edward G. Walter, Modesto, Calif.

Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

Filed: May 31, 1973 Appl. No.: 365,658

U.S. Cl. 250/281, 250/299 Int. Cl. H0lj 39/34 Field of Search 250/299,300, 283, 294

References Cited UNITED STATES PATENTS 3/1966 Wiley 250/299 4/l967Oehser 250/300 Primary Examiner-Archie R. Borchelt Assistant ExaminerC.E. Church Attorney, Agent, or FirmJohn A. Horan; F. A. Rohertson;Clifton E. Clouse. Jr.

[57 ABSTRACT 10 Claims, Drawing Figures CRYOGEN PUMP AND RESERVOIRCOMPUTER PATENTEBRYZY m (812,354

CRYOGEN PUMP AND RESERVOIR- COMPUTER LU 25 8 Al IONS 0F 20 KEV gINCIDENT on @2 DYSPROSIUM DOPED 553 CuF 12 (Do 32 l l 14 5 l i L06 PMCOUNTS 3 Fig.2 3 O O O O O I (D m 2 O l D O O PARTICLE MASSTHERMOLUMINESCENT DETECTOR FOR MASS SPECTROMETER ORIGIN OF THE INVENTIONBACKGROUND OF THE INVENTION This invention pertains to an ion detectorfor a mass spectrometer, and more particularly, it pertains to athermoluminescent crystal as an integrating ion detector for a massspectrometer.

The usual ion detector in a high-resolution doublefocussing massspectrometer is a photoplate which is located in a vacuum area of thespectrometer. The photoplate must be removed from the spectrometer to bechemically developed and analyzed. Such removal requires exposing vacuumareas of the spectrometer atmosphere. Thereafter these areas must bepumped down to restore the vacuum. Prior to each pump-down a newphotoplate is replaced in the spectrometer. The new photoplate tends tooutgas and thereby prolong the pump-down time. The typicalphotoplate-equipped spectrometer is thus limited to analyzing only a fewsamples per day. Another disadvantage of the photoplate detector is itsrelatively limited dynamic range about 100. This means that the highestion exposure still in the linear response range of the photoplate isonly 100 times greater than the lowest detectable exposure. Ideally adetector should have a dynamic range much greater than this so as toprovide a linear response over a very wide exposure range, thuseliminating the need for running either multiple calibration curves orreplicate exposures per sample.

SUMMARY OF THE INVENTION In brief, the present invention pertains to anion detector for a mass spectrometer, including means for establishing amagnetic field of specific strength; means for injecting an ion beam ofvarious charged particles to pass through the field at right angles tothe lines of the field, thereby causing the charged particles to followdivergent curved paths according to the chargemass ratio of theparticles, each of the paths terminating at a focal point in a focalplane; a dielectric crystal positioned in the focal plane of theparticle paths so that the charged particles impinge on the crystal atrespective focal points to create trapped electron-hole pairs in thecrystal, the number of pairs at each point being proportional to thetotal exposure time of the crystal to charged particles impinging atthat point and thus the number of particles; means for heating thecrystal to cause recombination of the electron-hole pairs to causethermoluminescent emission from the exposed areas; means for transducingthe thermoluminescence of each area to electrical pulses; and means forcounting the electrical pulses-corresponding to each area to determinethe total exposure of each area and hence the mass spectrum of the ionbeam.

It is an object of the invention to obtain a mass spectrum of an ionbeam injected into an evacuated space without disturbance of the vacuum.

Another object is to provide an ion detector with a dynamic rangegreater than Another object is to determine successive mass spectrumsrapidly and efficiently.

Another object is to provide an ion detector with increased sensitivity.

Other objects and advantageous features of the invention will beapparent in a description of a specific embodiment thereof, given by wayof example only, to enable one skilled in the art to readily practicethe invention which is described hereinafter with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional diagram ofa spectrometer in which an ion detector, according to the invention, ismounted.

FIG. 2 is a family of typical log-log curves illustrating the linearityand dynamic range of the ion detector of FIG. 1.

FIG. 3 is a representative general sensitivity curve that may be derivedfrom the curves of FIG. 2 and is useful for analyzing unknown beams.

. DESCRIPTION OF AN EMBODIMENT Referring to the drawing there is shownin FIG. 1 a mass spectrometer 11 in which an ion beam 13 is injectedbetween energy limiting electric sectors 15 into a magnetic field 17produced by a magnet 16. The ion beam is injected at right angles to themagnetic field which causes the beam to diverge and follow curved paths18 according to the charge-mass ration of the various ions of which thebeam is composed. A dielectric crystal 19, conveniently a singleelongated crystal, is positioned in the focal plane of the divergentbeams and is comprised of successive continuous areas on which thedivergent beams impinge, each area being representative of ions of aparticular energy and charge-mass ratio for a magnetic field of specificstrength. Trapped electron-hole pairs are created in the crystal at eachfocal point in a number that is proportional to the total exposure timeof the crystal to the portion of the beam impinging at each point.

, When it is desired to determine'the number of particles that hasimpinged on the various areas of the crystal, an electric heatingelement such as a wire 21, which is mounted adjacent and in brushingcontact with the crystal 19, is electrically heated and then moved overthe surface of the crystal by means of a stepping motor 23 and flexiblebelt 24, causing successive areas of the crystal to heat and luminesceas the wire is stepped over the crystal. A bank of photo-tubes 25 aremounted adjacent the crystal 19 with all of their outputs connectedtogether to a computer 27. The steps of the stepping motor may becorrelated with the mass of the particles impinging on each area of thecrystal, each step corresponding to a particular mass. The motor is alsoconnected to the computer which is programmed to accumulate the numberof counts from the phototubes for each step of the motor and totranslate the counts for a particular mass to coulombs. After the wire21 has been moved over the entire crystal, the computer totals togetherthe coulomb exposure of all areas and then prints out as a fraction ofthe total exposure the amount of exposure of each area corresponding toeach step of the motor. Such a printout, therefore, is a directindication of the mass spectrum of the beam 13. The wire 21 is returnedto its original position and the spectrometer is ready for the nextanalysis.

In operation, the spectrometer 11 may be calibrated with known beams toascertain the areas on which known ions will impinge and to obtain afamily of curves 26 as shown in FIG. 2, where each area of the crystalcorresponds to one of the curves, the curves of greater slopecorresponding to particles of smaller mass. A general sensitivity curve28 such as shown in FIG. 3 may be derived from any two or more curves 26by picking a number of counts such as and plotting the coulombs/numberof counts as a function of the mass of the particle. The curve 28 may beprogrammed into the computer so that at each step of the motor anadjusted value of coulombs per 1,000 counts is used to determine thecorrect exposure in coulombs at each step. Since the curves 26 arelinear, the curve 28 is also linear and may be used for all masses.Thereafter, all ion beams may be analyzed without further calibration ofthe spectrometer as long as the energy of the beam 13 and the strengthof the field 17 are maintained at the levels maintained duringcalibration or at adjusted levels that provide identical ion deflection.

The sensitivity of detection of the crystal 19can be increased bymaintaining the crystal at a cryogenic temperature to prevent emptyingof shallow lattice traps which otherwise empty at room temperature. Suchcooling may be accomplished by circulation of liquid nitrogen fromacryogen pump and reservoir 29 through cooling coils 31 around thecrystal l9.

Vagious types of dielectric crystals may be used for the crystal 19, forexample fused quartz. The structural strength of fused quartz inaddition to its commercial availability in large sizes and the energydepth of its lattice traps make it a desirable crystal. However, manyother crystals maybe used as well.

In a practical embodiment of the invention, a beam of Al* ions weredirec ted t o impinge on adielectric crystal ot dysptdsiu ni-dopedcalcium fluoride. The crystal was heated and a log-log plot of totalexposure to the ion beam versus the photomultiplier counts is shown inFIG. 2. The plot indicates a linear dynamic range of 10 (10 coulombs/10*coulombs).

While an embodiment of the invention has been shown and described,further embodiments or combinations of those described herein will beapparent to those skilled in the art without departing from the spiritof the invention.

What I claim is:

l. A mass spectrometer including:

means for establishing a magnetic field within said spectrometer;

. means for injecting an ion beam of charged particles to pass throughsaid field at right angles to the field lines of said field, said fieldcausing the charged particles to follow divergent curved paths accordingto the charge-mass ratio of the particles, each of said pathsterminating at a focal point in a focal plane;

a dielectric crystal positioned in the focal plane ofthe particle paths,said charged particles impinging on said crystal at respective focalpoints to create trapped electron-hole pairs in the crystal, the numberof pairs at each point being proportional to the total exposure of thecrystal to charged particles impinging at that point;

means for heating said crystal to cause recombination of theelectron-hole pairs, said recombination causing thermoluminescentemission from the exposed areas;

means for transducing the thermoluminescence of each area to electricalpulses; and

computer means for counting the electrical pulses corresponding to eacharea to thereby determine the mass spectrum of the ion beam.

2. The spectrometer of claim 1 wherein said crystal is a singleelongated crystal.

3. The spectrometer of claim 1 wherein said crystal is fused quartz. I

4. The spectrometer of claim 1 whereinsaid crystal is dysprosium-dopedcalcium fluoride.

5. The spectrometer of claim 1 wherein said injecting means includes anelectric sector for passing ions of a selected energy into said magneticfield.

6. The spectrometer of claim 1 wherein said heating means includes meansfor sequentially heating successive areas of said crystal.

7. The spectrometer of claim 1 wherein said heating means includes awire, means for heating the wire, and means for moving the wire oversaid crystal to heat said crystal.

8. The spectrometer of claim 7 wherein said moving means is a steppingmotor, and said computer means is operable for counting the steps of themotor, for counting the number of electrical pulses corresponding toeach area, for correlating the number of pulses for each area with eachstep of the motor, for counting the total number of electrical signals,and for indicating the counts corresponding to each area as a fractionof the total counts from the crystal.

9. The spectrometer of claim 7 wherein said cooling means includes meansfor cooling said crystal to a cryogenie temperature with liquidnitrogen.

10. The spectrometer of claim 1 further including means for cooling saidcrystal to maintain shallow lattice traps in said crystal filled withbackground to increase the sensitivity of the crystal.

1. A mass spectrometer including: means for establishing a magneticfield within said spectrometer; means for injecting an ion beam ofcharged particles to pass through said field at right angles to thefield lines of said field, said field causing the charged particles tofollow divergent curved paths according to the charge-mass ratio of theparticles, each of said paths terminating at a focal point in a focalplane; a dielectric crystal positioned in the focal plane of theparticle paths, said charged particles impinging on said crystal atrespective focal points to create trapped electronhole pairs in thecrystal, the number of pairs at each point being proportional to thetotal exposure of the crystal to charged particles impinging at thatpoint; means for heating said crystal to cause recombination of theelectron-hole pairs, said recombination causing thermoluminescentemission from the exposed areas; means for transducing thethermoluminescence of each area to electrical pulses; and computer meansfor counting the electrical pulses corresponding to each area to therebydetermine the mass spectrum of the ion beam.
 2. The spectrometer ofclaim 1 wherein said crystal is a single elongated crystal.
 3. Thespectrometer of claim 1 wherein said crystal is fused quartz.
 4. Thespectrometer of claim 1 wherein said crystal is dysprosium-doped calciumfluoride.
 5. The spectrometer of claim 1 wherein said injecting meansincludes an electric sector for passing ions of a selected energy intosaid magnetic field.
 6. The spectrometer of claim 1 wherein said heatingmeans includes means for sequentially heating successive areas of saidcrystal.
 7. The spectrometer of claim 1 wherein said heating meansincludes a wire, means for heating the wire, and means for moving thewire over said crystal to heat said crystal.
 8. The spectrometer ofclaim 7 wherein said moving means is a stepping motor, and said computermeans is operable for counting the steps of the motor, foR counting thenumber of electrical pulses corresponding to each area, for correlatingthe number of pulses for each area with each step of the motor, forcounting the total number of electrical signals, and for indicating thecounts corresponding to each area as a fraction of the total counts fromthe crystal.
 9. The spectrometer of claim 7 wherein said cooling meansincludes means for cooling said crystal to a cryogenic temperature withliquid nitrogen.
 10. The spectrometer of claim 1 further including meansfor cooling said crystal to maintain shallow lattice traps in saidcrystal filled with background to increase the sensitivity of thecrystal.